Accelerating a moving object with force

  • Thread starter Thread starter magnethead494
  • Start date Start date
  • Tags Tags
    Accelerating Force
AI Thread Summary
The discussion focuses on calculating the acceleration of a moving object using physics formulas, specifically F = MA and kinematic equations. Initial calculations suggest high acceleration values for a 300HP, 700-pound machine, with detailed breakdowns of acceleration per gear and corresponding distances. However, the calculations yield results that are inconsistent with expected distances and times, indicating potential errors in the application of formulas. Participants highlight the neglect of air resistance and the assumption of constant power as possible factors affecting accuracy. The conversation emphasizes the need to identify calculation mistakes to reconcile the discrepancies observed in the results.
magnethead494
Messages
28
Reaction score
0
Spreadsheet of data: http://i1105.photobucket.com/albums/h355/magnethead494/Screenshot2012-05-14at103445PM.png

Example dyno graph: http://i1105.photobucket.com/albums/h355/magnethead494/turbo_busa_dyno_hahn_stg1.jpg

I'm familiar with F = MA/G => F*G/M = A for english units (G = 32.17).

This works for accelerating from a standstill (IE, approx'ing to top of first gear),

160 ft-lb * 1.596 primary * 2.615 first gear * 4.0625 chain drive = 2,712 ft-lb at the axle
2,712 * 12 inches / 13.25 inch rolling radius = 2,456 pound force
2,456 pound-force / 700 pound-weight = 3.50 G-forces = 112.87 feet per second squared average acceleration

I know that seems like a high acceleration, but for a 300HP 700 pound machine, that's a 2.3 pound-per-HP ratio.

And the formula

(Vf)2 = (V0)2 + 2ad

for subsequent gears... But I don't know x.

Applying that formula to previous calculation,

(75.68 feet/sec)2 = (0MPH)2 + 2 * 112.87 fps2 * d

5,727.4624 feet2 per second2 = 225.74 fps2 * d

5,727.4624 feet2 per second2 / 225.74 feet per second2 = 25.372 feet to the top of first gear (AKA 25.372 feet to 51.6 miles per hour, at average 3.5G's)But how can I calculate the acceleration for subsequent gears?
I'm going to state my 3-part hypothesis..am I right or wrong?

Part 1: Using F=Ma to get acceleration per gear

Expanded:

top of gear:
primary * chain = 6.48375
rolling radius multiplier = 0.88888
Engine Torque at 10,500 RPM: 160 ft-lb
Composite value = 160 * 6.48375 * 0.8888888 = 922.13333
1st: 922.1333 * 2.615 / 700 * 32.17 = 110.82 feet per second squared (3.44G)
2nd: 922.1333 * 1.937 / 700 * 32.17 = 82.08 feet per second squared (2.55G)
3rd: 922.1333 * 1.526 / 700 * 32.17 = 64.67 feet per second squared (2.01G)
4th: 922.1333 * 1.285 / 700 * 32.17 = 54.45 feet per second squared (1.69G)
5th: 922.1333 * 1.136 / 700 * 32.17 = 48.14 feet per second squared (1.50G)
6th: 922.1333 * 1.043 / 700 * 32.17 = 44.20 feet per second squared (1.37G)

Part 2: Getting the distance per gear

1st: 75.682 = 02 + 2 * 110.82 * d => 5,727.46/221.64 = 25.84ft
2nd: 102.182 = 75.682 + 2 * 82.08 * d => 10,440.75 - 5,727.46 = 164.16 * d => 4,713.29 / 165.16 = 28.53ft
3rd: 129.712 = 102.182 + 2 * 64.67 * d => 16,824.68 - 10,440.75 = 129.34 * d => 6,383.93 / 129.34 = 49.35ft
4th: 154.032 = 129.712 + 2 * 54.45 * d => 23,725.24 - 16,824.68 = 108.90 * d => 6,900.56 / 108.90 = 63.36ft
5th: 174.242 = 154.032 + 2 * 48.14 * d => 30,359.58 - 23,725.24 = 96.28 * d => 6,634.04 / 96.28 = 68.90ft
6th: 189.77 = 174.242 + 2 * 44.20 * d => 36,012.65 - 30,359.58 = 88.40 * d => 5,653.07 / 88.40 = 63.95ft

25.84 + 28.5 + 49.35 + 63.36 + 68.90 + 63.95 = 299.9 feet (should equal something near 660ft)

Something is clearly wrong with the calculation...but what is it? The G forces seem realistic, the MPH calculations (spreadsheet and google to FPS calcs) seem correct, what else could be wrong?

Part 3: getting the time per gear

d = t/2 * (Vf+V0)

1st: 75.682 = 02 + (2 * 110.82 * t/2 * (75.68 + 0)) => 5,727.46 = 8,386.86 * t => 0.683 seconds
2nd: 102.182 = 75.682 + (2 * 82.08 * t/2 * (102.18 + 75.68)) => 10,440.75 = 5,727.46 + 14,598.75t => 0.323 seconds
3rd: 129.712 = 102.182 + (2 * 64.67 * t/2 * (129.71 + 102.18)) => 16,824.68 = 10,440.75 + 14,996.33t => 0.426 seconds
4th: 154.032 = 129.712 + (2 * 54.45 * t/2 * (154.03 + 129.71)) => 23,725.24 = 16,824.68 + 15,449.64t => 0.447 seconds
5th: 174.242 = 154.032 + (2 * 48.14 * t/2 * (174.24 + 154.03)) => 30,359.58 = 23,725.24 + 15,802.92t => 0.420 seconds
6th: 189.77 = 174.242 + (2 * 44.20 * t/2 * (189.77 + 174.24)) => 36,012.65 = 30,359.58 + 16,089.24t => 0.351 seconds

0.683 + 0.323 + 0.426 + 0.447 + 0.420 + 0.351 = 2.65 seconds (should equal something near 4.95 seconds)

Again, something is clearly wrong. I have a feeling it's in the main equation...(Vf)2 = (V0)2 + 2ad

A = G * F/m

d = t/2 * (Vf+V0)

If someone could point my mistakes, it would be appreciated.
 
Physics news on Phys.org
You've neglected air resistance. That would be pretty significant long before you reach top speed.
 
haruspex said:
You've neglected air resistance. That would be pretty significant long before you reach top speed.
One would think so, but it's surprisingly not as big an issue as one would think. The body panels and ground effects are done up to where, once you have the power on tap, air resistance is a very small loss, maybe 4% on MPH.

Either way, that's nothing to do with the question I asked. I asked where my calculations were off by 200% or so.
 
Last edited:
Then you're assuming constant power (300HP) and no drag.
In that case we can skip the details and go straight to the energy.
A 700 pound (318kg) mass at 189.77 fps (57.86 mps, 129Mph) has 532.5kJ.
A power of 300HP (225000 W, or thereabouts) delivers that in 2.37 seconds.
If that is not reflected in reality then you must be ignoring a significant loss somewhere.
 
The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?

Similar threads

Replies
6
Views
2K
Back
Top